Method for isolation and identification of Escherichia coli 0157:H7 and plating media for said process

An isolation plating medium for use in processes for the presumptive identification of Escherichia coli 0157:H7 from a sample that also contains other strains of Escherichia coli. The plating medium comprises at least one carbohydrate that Escherichia coli 0157:H7 is incapable of fermenting, but other strains of Escherichia coli do ferment said carbohydrate, a pH indicator dye that changes the color of the plating medium to a first color when the pH of the medium changes, a chromogenic substrate that reacts to beta-galactosidase to form a precipitate in the plating medium of a second color which contrasts with the first color. Both Escherichia coli 0157:H7 and the other strains of Escherichia coli are beta-galactosidase producers during metabolism, whereby a microorganism which ferments the carbohydrate but does not produce beta-galactosidase will produce colonies in the plating medium of the first color, Escherichia coli 0157:H7 which does not ferment the carbohydrate but produces beta-galactosidase will produce colonies in the plating medium of the second color, and the other strains of Escherichia coli which ferment the carbohydrate medium and produce beta-galactosidase will produce colonies in the plating medium of a third color which is the color that results from the mixing of the first and second colors, and a sufficient mass of an agent to solidify the mixture. The invention also includes processes of using the plating medium to identify and enumerate Escherichia coli 0157:H7 and other microorganisms.